Main menu

You are here

Mercury Retrograde Rotation

An observer on Mercury witnesses asn odd phenomenon with backward motion of the Sun in the sky of Mercury as Mercury passes through its closest point to the Sun. Mercury's orbit around the Sun is not circular, but elliptical, with its orbit having a closest point to the Sun (the perigee or perihelion) and the furthest point (the apogee or aphelion). One of the basis laws of gravitational motion is Kepler's 2nd law: "A line joining a planet and the Sun sweeps out equal areas during equal intervals of time.". The implication of this is that a planet travels fastest in its orbit at perigee and slowest at its apogee. Mercury rotates on its spin access 1-1/2 times in its orbit around the Sun, 3 times in 2 orbits, or in fancy words, it is in a 3:2 spin-orbit resonance with the Sun. When an orbiting object is viewed from the central object of the orbit, such as viewing Mercury from the Sun, or the Moon from the Earth, and the object is rotating in the same direction as its orbit (the usual case), the observer sees one less spin rotation per orbit because the observer is rotating once to follow the orbit. An example of this is the Moon, where it actually rotates once per orbit in a 1:1 spin-orbit resonance with the Earth. For us on Earth, the Moon does not appear to rotate at all, with one face of the Moon pointing towards Earth at all times. In the case of Mercury viewed from the Sun, Mercury appears to rotate 1-1/2 subtract 1 times, or 1/2 time per orbit, or if you will, once for every two orbits. Here's where Mercury's perigee makes it interesting. When passing by this close point in the orbit, the speed of its orbital motion overcomes its spin rotation, and for a brief period of time Mercury appears to rotate backwards. From the perspective of someone on Mercury watching the Sun, the Sun travels slowly across the sky, slowing down, moving backward for a brief period of time, and then preceding on its normal path. Here is an animation of Mercury's rotation over two orbits as viewed from the Sun. Note the increased apparent size of Mercury at its perigee, the slight reverse rotation at this time, and the return of the same face of Mercury after 2 orbits as when the animation started.